The usual papermaking machine has three primary sections: a forming section, a press section and a drying section. In the forming section, wet pulp is deposited on the forming surface of a forming fabric which is of the nature of a fluid-permeable endless belt. Vacuum is applied to the underside of the forming fabric to withdraw moisture from the pulp, causing the pulp to form a sheet on the forming fabric. From the forming section, the sheet is transferred to the press seciton and is conveyed by a press fabric through a series of press rolls to further remove water from the web. From the press section, the web is then transferred to the dryer section where it is passed about a series of heated dryer cylinders. One or more dryer fabrics are employed to press the moist web uniformly and successively against the dryer cylinders to dry the web. As used herein and in the claims, the term "papermaking machine" is to be considered in a broad or generic sense, the machine producing a paper or paper-like material such as pulp, board, asbestos sheet or other similar structures.
In the dryer section, the dryer cylinders are internally heated by steam or the like. The cylinders usually have imperforate surfaces for contacting the paper web. Other rolls, such as pocket rolls may have surfaces which are perforated or slotted to permit the passage of heated air therethrough to increase the drying action on the web.
Several problems are encountered in the dryer section. First of all, it has been found that the surface temperature of the dryer cylinders may vary axially across the cross machine direction by as much as 40.degree. F. This produces the distinct possibility that some portions of the web will be subjected to greater drying action than other portions of the web, resulting in a non-uniform moisture profile across the web.
A second problem results from the fact that moisture from the paper web tends to collect in enclosed areas or "pockets" in the dryer section. Usually more moisture accumulates in the central portions of the pockets than in those portions of the pockets lying nearer the lateral edges of the dryer section, the portions nearer the lateral edges of the drying section being more readily ventilated by the surrounding atmosphere.
Thus, by virtue of the variance in temperature across the cross machine direction of the dryer cylinders and moisture accumulated in pockets in the dryer section, the web generally will have a higher moisture content near its center, than at its edges. This is undesirable from the standpoint of quality control of the end product. Prior art workers have taken numerous steps to alleviate this problem. First of all, dryer fabrics have been devised which are fluid-permeable, such dryer fabrics having an open weave. An exemplary open weave dryer fabric is taught in U.S. Pat. No. 2,180,054. Dryer fabrics in the form of nonwoven structures such as needled felts and perforated plastic belts have also been used. To further alleviate the problem, various types of air jet apparatus or exhaust means have been provided to eliminate moisture build-up in the dryer pockets. Such devices are generally extremely complex and difficult to maintain, particularly in new dryer sections capable of handling webs having a width of the order of 400 inches.
Yet another approach to the provision of a more uniform moisture profile across the web is taught in U.S. Pat. No. 3,867,766. According to the teachings of this reference, a fluid permeable dryer fabric is provided, the permeability of which varies at selected locations across its width. This is accomplished in several ways. In a first embodiment, the machine direction yarns in the edge regions of the dryer fabric are more closely spaced with respect to one another than in the center region of the dryer fabric. It is to be noted that the phrase "machine direction" used herein and in the claims refers to the direction of travel of the dryer fabric when mounted in the dryer section of the papermaking machine. The phrase "cross machine direction" refers to that direction transverse the dryer fabric and perpendicular to the direction of travel of the dryer fabric in the dryer section of the papermaking machine.
In another embodiment, U.S. Pat. No. 3,867,766 teaches selectively varying the diameter of the machine direction yarns so that those yarns which lie in the area desired to be of reduced permeability have a diameter greater than those machine-direction yarns located in the more permeable sections of the dryer fabric. In a third embodiment, this reference teaches the utilization of machine direction yarns having a higher bulk construction near the lateral edges of the dryer fabric. These yarns will tend to flatten and close off the effective open area between adjacent yarns, thus reducing the permeability of the dryer fabric near its edges. In a final embodiment, the dryer fabric is selectively treated with greater amounts of resin or the like at its edges, than in its center, to reduce permeability at the edges thereof.
The various approaches taught in the above mentioned U.S. Pat. No. 3,867,766, while effective, have certain drawbacks. For example, when the frequency of machine direction yarns is increased at the edges of the dryer fabric, the edges tend to resist stretch to a greater extent and operate at a higher tension than the center of the dryer fabric. Furthermore, as the machine direction yarns elongate and loose crimp, the cross machine direction yarns must accept crimp. However, the cross machine direction yarns are more difficult to bend as their ability to accept crimp is a function of the distance between the machine direction yarns. As a result, it has been found that dryer fabrics of the type taught in U.S. Pat. No. 3,867,766 demonstrate differences in machine direction physical characteristics across the width of the dryer fabric. These differences in machine direction physical characteristics between sections of differing end counts present problems in achieving uniform flatness across the width of the dryer fabric and, similarly, can give rise to difficulties in running flat and wrinkle free in the dryer section of the papermaking machine. Furthermore, it is generally accepted that higher operating tension in a dryer fabric gives rise to increased drying of the web. Therefore, dryer fabrics in which the edge portions have a higher frequency of machine direction yarns, to reduce the permeability and thereby the drying rate of the edge portions of the fabric, suffer some counteracting effects of increased drying of the web at the fabric edges due to their edges operating at a higher tension.
The use of one or more resin coatings on the dryer fabric to vary its permeability across its width has not proven very effective due to the difficulty of maintaining the resin coating throughout the life of the fabric. This is even more difficult to achieve in dryer fabrics made of monofilament yarns, due to the resistance of monofilament yarns to the acceptance of resin coatings.
The present invention is based upon the discovery that if, in a woven dryer fabric, additional cross machine direction yarns, which may be of various thicknesses or bulk, are located in the fabric at selected positions across the fabric, the permeability of the dryer fabric can be varied, by design, in different sections across its width. The additional cross machine direction yarns have little or no effect on the machine direction load bearing properties of the dryer fabric. Thus, there will be no substantial differences in machine direction physical characteristics between the sections of the dryer fabric having different permeabilities. Furthermore, the present invention can be practiced in the manufacture of dryer fabrics made substantially from monofilament yarns. The dryer fabrics of the present invention can also be pinseamed.
A well known problem encountered in the dryer section of a papermaking machine is frequently referred to as "edge flutter" in the web. This problem is more severe at the web edges and tends to limit the speed at which the dryer section of the papermaking machine can be run. The dryer fabrics of the present invention provide much better control of edge flutter in the web and therefore permit the dryer section to be run at higher speeds.
Another well known problem in the art is a malformation in the web edges known as "grainy edges" or "cockled edges" due to greater drying of the web at the edges and flutter. The dryer fabrics of the present invention tend to reduce or eliminate such malformations in the web edges.
Woven dryer fabrics are used in the dryer section of the papermaking machine in the form of endless belts. To this end, the dryer fabrics may be woven directly in the form of endless belts by endless weaving techniques well known in the art. In such an instance, the weft or filling yarns extend in the machine direction and the warp yarns extend in the cross machine direction. Alternatively, the dryer fabrics can be flat woven with their ends joined by well known means to form continuous belts. When the dryer fabrics are flat woven, the warp yarns extend in the machine direction and the weft or filling yarns extend in the cross machine direction. While the dryer fabrics of the present invention can be woven by either technique, for purposes of an exemplary showing the dryer fabrics will be described herein in terms of flat woven fabrics.